90 LECTURES TO SCIENCE TEACHERS. 



is true that in many of the more familiar ways of handling 

 it, the forces developed by its slight frictional qualities are 

 small compared to those due to its inertia, and it is there- 

 fore not surprising that those who theorised on the resist- 

 ance of ships thought it quite accurate enough to treat of 

 the effect of the inertia only, and to neglect the compara- 

 tively small frictional qualities. 



FIG. 1. ELEVATION. CROSS SECTION. 



It was assumed, then, for the purposes of calculation, that 

 the fluid, being frictionless, would offer no resistance to a 

 perfectly thin, flat, smooth plane, such as that shown in 

 Fig. 1, moving edgeways through it, since this would in no 

 way tend to set its particles in motion. But it is obvious 

 that a ship, or fish, or other body, such as that shown in 

 Fig. 2, moving through the water, has to be continually 

 setting the particles of water in motion, in order, first to 



FIG. 2. ELEVATION. CROSS SECTION. 



get them out of its way, and afterwards to close them 

 together again behind it, and that the inertia of the particles 

 thus set in motion will supply forces reacting against the 

 surface of the body. And it seemed certain, at first sight, 

 that these reactions or forces on the surface of the body 

 would necessarily so arrange themselves as to constitute 

 resistance. 



